1. Field of the Invention
The present invention relates to metallizable isotropic substrates comprising a polymeric material, reinforcing fillers and pulverulent fillers, as well as to a process for the preparation of such substrates.
2. Description of the Prior Art
In the electronics field, the trend towards high densities of integration in integrated circuits manifests itself, for example, by the need to have available rigid boxes equipped with a large number of outlets. The chip carriers are a suitable solution to this problem, but their development is restricted both by their high price and by as yet unsolved technical problems. For the moment, these ceramic boxes are often mounted on ceramic substrates and soldered thereto by means of metal contacts.
However, ceramic carriers are expensive, delicate to handle because of their fragile nature, and excessively small. Hence, there arises the problem of providing carriers which have a coefficient of thermal expansion close to that of silicon or of ceramics, which are not sensitive to moisture, which are isotropic and which have good mechanical properties both at low temperatures and at high temperatures which can be as much as 250.degree. C.
Numerous solutions to this problem have been proposed, some of which have already reached the market stage; they employ polymeric materials such as epoxy resins, polyimides or mixtures of epoxy resins and polyimides and reinforcing fabrics made from glass fibers, quartz fibers, boron fibers, carbon fibers or aromatic polyamide fibers. However, these substrates themselves exhibit a certain number of disadvantages. Thus, for example, the polyimide substrates reinforced with glass fibers have too high a coefficient of expansion. The polyimide substrates reinforced with Kevlar.RTM. (aromatic polyamide) fabric are difficult to prepare and do not permit good hole-piercing. Moreover, they are anisotropic.
It is on the other hand known, from French Pat. Nos. 2,256,452 and 2,163,383, that it is possible to produce non-woven articles which consist of webs of fibers which are infusible, or have melting points above 180.degree. C., bonded together by polymers of the polyamide-imide or polyimide type. However, the porous structure of these non-wovens as well as their mechanical properties are entirely different from the substrates according to the invention.